From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of pressure wave propagation and velocity within a fuel injection system.
| Identifier | ExFiles\Box 179b\3\ img106 | |
| Date | 1st February 1933 guessed | |
| -16- pressure in the system is built up. On the opening of the spill port, which is equivalent to the plunger being brought suddenly to rest, a negative pressure wave traverses the fuel line and the pressure falls. Now the velocity of propogation of the pressure wave equals the velocity of sound in the oil in the pipe. It therefore depends on the density and pressure of the fuel in the pipe. In normal injection systems an average value for this velocity is about 4800 ft/sec. The velocity of the plunger depends entirely on the cam form and pump speed. In the R.R. pump at 1000 R.P.M. it is 4 ft/sec. The fundamental relation between the velocity of the plunger and the magnitude of the pressure wave is P = (Bulk Modulus / Velocity of Sound Vg) x velocity of plunger where P is the magnitude (or amplitude) of the pressure wave, Vg is the velocity of sound in the pipe, and the bulk modulus is the bulk modulus of the oil in the pipe. It follows from this that the higher the plunger speed the greater is the pressure wave propagated along the pipe, but its velocity depends entirely on the initial pressure in the pipe. To simplify matters assume that the initial pressure in the pipe is constant at all speeds. Then for a given fuel injection system having a fuel pipe of length l, the time taken for the pressure wave to traverse the length of the pipe is l/Vg and hence is independent of pump speed. The magnitude of the wave will depend on the velocity of the plunger and hence on pump speed. Suppose the injector is set to open at 3000 lbs/sq.in. then if the initial pressure wave exceeds 1500 lbs/sq.in. (which at total reflection becomes 3000 lbs/sq.in.) the valve will open l/Vg secs. after suction port closes. If the plunger speed is not sufficient to produce a wave of this magnitude the wave will be totally reflected at the injector return to the pump, be reflected and again reach the injector 3l/Vg secs. after suction port seals. It is clear from this that the pump lag speed curve in its ideal form will be stepped. | ||